#1If you wait 5 - 10 minutes when Guybrush and Wally are hanging over the acid pit in LeChuck's fortress, they will be lowered into the acid pit.
You will get a second chance of course.

Added: Unknown

#2You can also try drowning Guybrush when he is diving for the Mad Monkey figurehead. Let him stand under water with the figurehead and wait for him to turn blue, drop the figurehead and raise to the surface.